An article entitled "Development of Diesel Engine for '94 U.S.A. Emission Regulations" on p47-p52 of "Newly Developed Engines" published by the Society of Automotive Engineers of Japan, Inc. in 1995 discloses reduction of premixture combustion and promotion of diffusive combustion to reduce nitrogen oxides (NOx) and particulate matter (PM) emitted by a direct injection diesel engine.
When premixture combustion is performed, fuel injection is performed far in advance of the compression top dead center of an engine piston, fuel is vaporized during the ignition delay period and fuel is burnt all at once.
Consequently, although discharge of PM is suppressed, as combustion occurs abruptly, the combustion temperature increases and NOx occurs in large quantities. Therefore, premixture combustion should be suppressed in order to reduce NOx. Various methods are used to achieve this, e.g., exhaust gas recirculation (EGR), retardation of fuel injection start timing, higher compression ratio, and reduction in size of the injection nozzle of the fuel injector.
Near compression top dead center, the temperature of the combustion chamber is high and the fuel which is injected near the compression top dead center is burnt immediately after injection. This type of combustion is called diffusive combustion, and as the fuel is not all burnt at once, there is little temperature rise due to the combustion and the discharge of NOx decreases.
However, as the injected fuel is burnt without being completely vaporized, mixing of fuel and air is not complete and the discharge of PM increases. Hence, to promote the flow of air in the combustion chamber so as to assist the mixing of fuel and air, a deep cavity is formed in a piston and the formation of squish flow by intake air is promoted.